This invention relates to a waterborne polyester paint. More particularly, it relates to a waterborne polyester paint useful as an intermediate coating to be applied between an electrodeposition coating layer and a top coating layer for forming a multilayer coating film on a metallic substrate such as automobile bodies.
Recently, from the standpoint of economizing natural resources and energy and for environmental reasons, much interest has been placed on waterborne paints. It is common practice in automobile finish to apply an electrodeposition coating layer, an intermediate layer of primer or anti-chipping coating and a color coating layer, successively. If a waterborne paint is used as the intermediate coating, not only it is possible to avoid the use of organic solvents, but also the intermediate coating can be applied on the electrodeposition coating layer wet-on-wet and then baked the two layers baked simultaneously to reduce the number of steps.
Water-soluble or water-dispersible polyester resins have been conventionally used for such intermediate coatings as the vehicle resin thereof. The term "polyester resins" as used herein includes oil-free polyester resins and alkyd resins. The water-soluble polyester resins known in the prior art have a problem that the solution viscosity thereof increases as the degree of neutralization with a base increases. Therefore, if the viscosity is adjusted to a suitable level for application, the solids content of the paint necessarily decreases so that workability such as anti-sagging properties is greatly impaired. Conversely, if attempts are made to obtain satisfactory workability while retaining relatively high solids content by decreasing the degree of neutralization, the dispersion stability of the resin will be impaired. Since the viscosity is proportional to the resin molecular weight, satisfactory workability would be achieved even at a solids content and a degree of neutralization sufficient to exhibit satisfactory dispersion stability by using relatively low molecular weight resins. However, satisfactory film performance will not be obtained by the use of such low molecular weight resins.
Therefore, a need exists for a water-dispersible polyester resin for use as vehicle resins for intermediate coating formulations of which satisfactory workability and film performance are compatible.